The present invention relates to improvements in element construction and control for electric immersion liquid heaters.
Electric immersion liquid heaters are commonly found in kettles, jugs, urns, pans and the like. Such implements generally comprise a vessel adapted to contain a liquid to be heated, an electrically heated element located generally within a lower part of the vessel with power being supplied to the element through the wall of the vessel, and some form of control means which is sufficiently sensitive to detect element overheating in a dry or dry boil condition within the vessel so that power is disconnected from the element to avoid damage to the element and associated parts of the implement. With those types of implements where it is commonly desired to have an automatic turn off feature (e.g. kettles and jugs) upon boiling, then the control means will normally also include some form of resettable thermally activated means for sensing a boil condition and turning off the power to the element in response thereto, the means being capable of being reset for reuse of the implement. Typically this is achieved by using a resettable bi-metal switch onto which steam is directed when a boil condition exists in the liquid heating vessel. The location of the bi-metal switch may vary with common locations being either in the upper regions of the vessel or at a lower region adjacent the power connection to the heating element.
There are numerous designs or arrangements of implements of the aforementioned kind. One commonly used arrangement comprises an element formed in two loops in a W type format with the two outer legs (cold tail ends) projecting outwardly through a central plate and within the vessel being brought inwardly in a loop with the loop secured to the central plate by soldering. The point of soldering to the central plate being above the level of the two outer legs and this region is termed the "hot return" section of the element as it is normally the first part of the element to be exposed to air when liquid level drops (e.g. in a boil dry solution). The central plate is commonly circular and is located to overly a relatively large aperture in the vessel wall. Appropriate seals are required of sufficiently large diameter to seal the aperture in the vessel wall. On the outer side of the aperture a switch body is mounted (which is also sealed by suitable seals around the aperture) and includes appropriate control elements which commonly include a main boil dry control element sensing the temperature of the "hot return" section of the element by engagement with the rear wall of the central plate in the vicinity of the soldered connection of the "hot return" section to the central plate. One arrangement of the aforementioned kind is shown, for example, in Australian Patent No. 492533.
More recent, Australian Patent Specification Nos. 22438/92 and 51548/90 disclose arrangements where the heating element protection against excessive temperatures caused by dry operation is achieved by temperature responsive bimetallic switches sensing temperature of the cold tail ends of the resistance heating element outwardly of the vessel thereby avoiding the need to have a hot return portion of the element soldered to the central plate. The specifications, however, still use a large central plate through which the cold tail ends project.
While arrangements of the kind disclosed in Australian Patent No. 492533 have been used for some years and have operated quite satisfactorily, they do, however, have a number of disadvantages in practice. Firstly, the arrangement requires a relatively large opening in the vessel wall which then requires seals of a relatively large diameter which are expensive. Secondly heat sensing off the back of the central plate requires that the boil dry switch (commonly a bi-metal switch) be mounted with critical tolerances from the backing plate, that is the control unit must be critically positioned for correct operation. Thirdly, it is usual to mount from the central plate mounting spigots with threaded ends that are adapted to pass through bores in the control unit with nuts being threaded onto the outer ends of the spigots to hold the central plate (and heating element) to the control unit in the desired configuration. This, however, leads to significant manufacturing difficulties particularly in relation to maintaining tolerances on the heating element and central plate sub assembly which is not commonly made by the same supplier as the control unit. Fourthly, the resettable control (when used) to switch off power when liquid boils in the vessel can be mounted either with the main control unit or at some remote location. If it is mounted with the main control unit then there is commonly a complicated steam delivery path to the main control unit. Alternatively, if it is mounted at a remote location then there is a complication of separate mounting and wiring requirements. All the foregoing add to the difficulty and cost of assembling a liquid heating implement of this general type.
The objective of the present invention is to provide improvements in element construction and control for electric immersion liquid heaters which will avoid or minimize at least some of the aforementioned difficulties.